1. Technical Field
The present invention relates to the gravel packing of wells and in one of its aspects relates to a method and apparatus for gravel packing long intervals of a well.
2. Background of the Invention
In producing hydrocarbons or the like from certain subterranean formations, it is not uncommon to produce large volumes of particulate material (e.g. sand) along with the formation fluids. The production of this sand must be controlled or it can seriously affect the economic life of the well. One of the most commonly-used techniques for sand control is one which is known as xe2x80x9cgravel packingxe2x80x9d.
In a typical gravel pack completion, a screen or the like is positioned within the wellbore adjacent the interval to be completed and a slurry of particulate material (i.e. xe2x80x9cgravelxe2x80x9d), is pumped down the well and into the annulus which surrounds the screen. As liquid is lost from the slurry into the formation and/or through the screen, gravel is deposited within the annulus to form a permeable mass around the screen which, in turn, permits produced fluids to flow into the screen while substantially screening out any particulate material.
A major problem in gravel packing, especially where long or inclined intervals are to be completed, is insuring that the gravel will be distributed throughout the completion interval. That is, if gravel is not distributed over the entire completion interval, the gravel pack will not be uniform and will have voids therein which reduces its efficiency.
Poor distribution of gravel across an interval is often caused by the premature loss of liquid from the gravel slurry into the formation as the gravel is being placed. This loss of fluid can cause the formation of xe2x80x9csand bridgesxe2x80x9d in the annulus which, in turn, block further flow of the slurry through the well annulus thereby preventing the placement of sufficient gravel (a) below the bridge in top-to-bottom packing operations or (b) above the bridge, in bottom-to-top packing operations.
To alleviate this problem, xe2x80x9calternate-pathxe2x80x9d well tools (e.g. well screens) have now been developed which provide good distribution of gravel throughout the entire completion interval even when sand bridges form before all of the gravel has been placed. In alternate-path well tools, perforated shunt tubes extend along the length of the tool and receive gravel slurry as it enters the well annulus which surrounds the tool. If a sand bridge forms in the annulus, the slurry can still flow through the perforated shunt tubes to be delivered to different levels in the annulus above and/or below the bridge to thereby complete the gravel packing of the annulus. For a more complete description of various alternate-path well tools (e.g.. gravel-pack screens) and how they operate, see U.S. Pat. Nos. 4,945,991; 5,082,052; 5,113,935; 5,515,915; and 6,059,032; all of which are incorporated herein by reference.
Alternate-path well tools, such as those described above, have been used to gravel pack relatively thick wellbore intervals (i.e. 100 feet or more) in a single operation. In such operations, the carrier fluid in the gravel slurry is typically comprised of a highly-viscous gel (i.e. greater than about 30 centipoises). The high viscosity of the carrier fluid provides the flow resistance necessary to keep the proppants (e.g. sand) in suspension while the slurry is being pumped out through the small, spaced openings along the perforated shunt tubes into the different levels of the annulus within the completion interval. However, as recognized by those skilled in the art, it is often advantageous to use low-viscosity fluids (e.g. water, thin gels, or the like; about 30 centipoises or less) as the carrier fluid for the gravel slurry since such slurries are less expensive, do less damage to the producing formation, give up the gravel more readily than do those slurries formed with more viscous gels, and etc.
Unfortunately, however, the use of low-viscosity slurries may present some problems when used in conjunction with xe2x80x9calternate pathxe2x80x9d screens for gravel-packing long, inclined, or horizontal intervals of a wellbore. This is primarily due to the low-viscosity, carrier fluid being prematurely xe2x80x9clostxe2x80x9d through the spaced outlets (i.e. perforations) in the shunt tubes thereby causing the shunt tube(s), themselves, to xe2x80x9csand-outxe2x80x9d at one or more of the perforations therein, thereby blocking further flow of slurry through the blocked shunt tube. When this happens, there can be no assurance that slurry will be delivered to all levels within the interval being gravel packed which, in turn, will likely produce a less than desirable gravel pack in the completion interval.
The present invention provides a well tool and method for gravel packing a long or inclined completion interval of a wellbore wherein the gravel is distributed throughout the interval even when using a low-viscosity slurry. Basically, a well screen having the slurry distribution system of the present invention thereon is lowered into the completion interval on a workstring. The slurry distribution system is comprised of a plurality of intermediate manifolds which are spaced along the length of screen and which are fluidly connected together. Slurry, which is comprised of a low-viscosity carrier fluid (e.g. water) and a proppant (e.g. sand), is pumped down the wellbore and is fed into the first intermediate manifold.
Where the well screen is to be used to complete an interval in a substantially vertical wellbore, the slurry may be supplied to the first intermediate manifold through at least one feed tube which is open at its upper end. Where the well screen is to be used to complete an interval in a substantially horizontal wellbore, a supply manifold may be provided which is fluidly connected to the first intermediate manifold by at least one feed tube and which receives slurry directly from a cross-over or the like in the workstring.
Each intermediate manifold has at least one upper shunt tube which extends upward therefrom and at least one lower shunt tube which extends downward therefrom. If a supply manifold is present, it will have only downward shunt tube(s) extending therefrom. Each shunt tube is perforated with a plurality of exit openings that are spaced along the outer length of the tube. A length (e.g. from about 2 feet to about xc2xd of the entire length of the tube) of each tube is preferably left blank (i.e. without openings) from the inlet end. This creates turbulent flow and prevents fluid loss from the slurry as it flows into a shunt tube thereby keeping the proppants in suspension until they exit the tube through the openings therein.
As the slurry fills the first intermediate manifold, it will flow substantially simultaneously upwardly through the upper shunt tube and downwardly through the lower shunt tube and will exit the respective tubes into zones which are spaced from each other within the annulus surrounding the screen.
The slurry then flows through a feed tube from the first intermediate manifold into a second manifold from which the slurry again flows both upward and downward substantially simultaneously through the respective shunt tubes, fluidly connected to the second intermediate manifold, and out the openings therein into different zones spaced from each other within said annulus. By overlapping the openings in a lower shunt tube of an upper manifold with the openings of an upper shunt tube of a lower manifold, slurry will be delivered to the complete interval which lies between the two respective manifolds. By providing sufficient intermediate manifolds to extend throughout the interval to be completed, gravel will be distributed to all zones within the interval even when using a low-viscosity slurry and/or if a sand bridge should form within the annulus before the gravel pack is complete.